Rolling Ammo Master

This one is pretty straight forward. You should be able to unlock this Trophy in a single playthrough. However, there is an easy method if you're struggling to obtain this Trophy.

When you get to the courtyard after climbing up the well in the Chateau (Chapter 6, after going through the cave), you'll encounter a group of enemies. First, clear the area, then you should be able to loot an Arm Micro from one of your fallen foes. Simply roll over and pick it up. Now you'll drop your .45 Defender. Repeat the process: pick up Arm Micro, then the .45 Defender, then the Arm Micro, etc. Do this 20 times and you'll earn the Trophy.

You can do this in any place, just make sure you have two of the same type of weapon (Ie. AK 47 and RPG, AK 47 and M9, Para 9 and Arm Micro) available. Picking up a gun of the same caliber will replace the one you have and you'll automatically drop the gun on the ground. Give yourself some space, because you may accidentally take cover by pressing Circle.